[Mesa-dev] [PATCH] egl/wayland: Set __DRI_IMAGE_USE_SCANOUT for shared buffers

[Axel Davy]

Hi,

On 27/01/2016 13:43, Daniel Stone wrote:
> Hi,
>
> On 27 January 2016 at 09:34, Michel Dänzer <michel at daenzer.net> wrote:
>> The compositor may have the hardware scan out directly from the buffers
>> sent by the client, so we must make sure the buffers we create are
>> suitable for scanout.
> If the compositor wants to scan out directly, it will import via GBM,
> which is in a position to reject the import if the buffer is not
> suitable for scanout. So there's something missing here, either in the
> GBM implementation to set magic flags when importing, or failure to
> communicate tiling mode correctly, or whatever.
>
> So for now, I would NAK this and fix the underlying problem, before
> forcing every client buffer to be scanout-capable, which can cause
> performance issues of its own. This also introduces a
> resource-contention issue (e.g. if the display controller can only
> scan out from physically-contiguous memory); you'll cause all client
> buffer allocations to fail unless it can allocate a potentially
> contended resource.
>
> Please fix the radeonsi/Gallium implementations of GBM and/or wl_drm
> to deal with tiling/compression correctly instead.
>
> Cheers,
> Daniel
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Scanout buffers are already used for DRI2 and DRI3 by default (for Mesa 
drivers),
and it looks like a sane default behaviour given there is no current
way to know if the compositor would like to use the buffer as scanout
or not.

As long as the app has no way to know what the compositor wants (there
was talks I remember on irc on ways to do that), I think enforcing scanout
as default is sane.

The other thing you mention, ie failing to import non-scanout buffers 
when scanout is requested is, I believe,
unrelated to this patch, and would require a bit more work. In 
particular this is handled by a function validateUsage,
which doesn't seem to be fully implemented on gallium. I suggest to file 
a bug on bugzillla.

Yours,

Axel